This invention concerns generally the field of cellular communication for voice and packet data and in particular cellular communication systems utilising wide beam antennas and narrow beam antennas.
Narrow beam base station antennas are widely used in for instance TDMA (Time Division Multiplex Access) systems and CDMA (Code Divison Multiplex Access) systems for providing a better spectrum utilisation.
Very narrow lobes can be accomplished by phase array antennas, which moreover may steer lobes in desired directions.
The frequency reuse distance within a cellular radio communication system is dependent on the C/I requirement (Carrier to Interference) for the deployed system. The C/I performance is dependent on modulation, decoding and information coding of the transferred information. The cross interference within a TDMA system originates from adjacent base stations within the frequency reuse pattern utilising the same frequencies. The reduction of cross interference and the cellular systems overall sensitivity to cross interference determines the reuse pattern. A tighter reuse pattern entails a higher cellular system capacity as is known in the prior art. In CDMA systems, the cross interference originates both from other users within the cell and from adjacent cells.
Thus, a typical remedy for dealing with increasing traffic in cellular networks is to introduce narrow beam antennas in base-stations having omnidirectional antennas thereby splitting the cell in sectors. Moreover, a cell, which already has narrow beam antennas, can be further sectorised by introducing even narrower beam antennas.
In typical systems, narrow beam antennas are not deployed in mobile terminals since such antennas are too complex for mobile terminals. However, using narrow beam base station receiving antennas also reduce uplink interference from mobile terminals since the mobile output power can be reduced due to the improved antenna gain.
In the following, we will use the terms narrow and wide beam in their comparative sense, i.e. a three sector antenna is a narrow beam in comparison to an omni-cell antenna and a five sector antenna is a narrow beam in comparison to a three sector antenna.
In the typical migration scenario, wide and narrow beam antennas will co-exist. Since the narrow beam antennas add complexity to the system and require resources from the base station, the channel capacity will typically be limited on the narrow beam antennas, whereby only a subset of the total traffic can be deployed on the narrow beam antennas. As an example, in a three sector base station each sector could be equipped with one phase array antenna dividing the sector into narrow beams in order to decrease the cross interference level. This technique has been further described in U.S. Pat. No. 5,848,358.
This raises the issue, which traffic channels shall be given priority to be deployed on narrow beam antennas.
The distance between the base station and the mobile terminal is one of the most important criteria for narrow beam intra cell hand-over since a higher output power level creates more interference. Moreover, since mobile terminals near the cell boundary tend to interfere with neighbour cells, a narrow-beam intra-cell hand-over from a wide beam to a narrow beam is often appropriate for terminals located far from the base-station.
In known systems, omni or three sector antennas covering the whole cell are for instance used in the initial part of a transmission and on broadcast channels. Depending on the characteristics of the transmission, an intra cell hand-over will be carried out in order to move the transmission to a narrow beam antenna.
According to prior art EP-A-0 526 436, the speed of a mobile terminal is a parameter, which is used for handover decisions. According to this document, fast moving mobile terminals are assigned to so-called umbrella cells, while stationary ones are assigned to micro cells.
Prior art document EP-A-0 926 843 shows a base-station arrangement having both omni directional antennas and narrow beam antennas. According to this solution, a connection shall be allocated to a narrow beam antenna, if this connection requires a higher QoS i.e. higher bit rate, in order to improve the link budget. This provides the opportunity for using a higher order of modulation or decreased correction coding which entail an increased throughput.
Prior art document U.S. Pat. No. 5,848,358 discloses various methods for performing intracell handover, i.e. handover between channels within a base station, whereby the cell is sectorised using array antennas.
According to one embodiment of the above method, a candidate mobile station for intracell handover is classified into power classes and spatial sectors. The channel allocation is carried out, such that mobile stations with approximately equal power levels and well separated sectors share the same channel.
According to another embodiment, the relative up-link interference on the mobile station and the relative up-link interference on other mobiles from the candidate mobile station as well as the downlink interference to existing downlinks are computed. From this information, the best channel and sector is chosen to a particular intra cell handover candidate.
The present invention seeks to provide a new method of optimising performance for cellular mobile telephone or data systems or a combination of telephone and data systems in which, wide beam and narrow beam antennas are used in combination.
This object has been provided by the subject matter defined by claim 1.
It is a further object of the invention to accomplish a method in which the interferences in a cellular system can be minimised.
This object has been accomplished by the subject matter according to claim 2.
It is a further object to accomplish an enhanced efficiency of reducing interferences in a cellular system.
This object has been accomplished by the subject matter according to claim 3.
It is a further object to set forth a method for reducing interferences in a cellular system, which is highly decentralised and is not adding any network control signalling between base stations and the network management system.
This object has been accomplished by the subject matter defined by claim 4. This object has also been accomplished according to respective claims 5 and 6.
Further advantages will appear from the following detailed description of the invention.